Development of hierarchical or three-dimensional structures at the sub-micrometer scale is becoming increasingly important with technological advances in microelectromechanical and nanoelectromechanical systems, microfluidic devices, microoptics or nano-optics, toolsets for biologists (microfluidic chips for deoxyribose nucleic acid array), and medicine (microsurgical tools).
Hierarchical structures are responsible for some unique properties in the natural world such as the extreme surface hydrophobicity of a lotus leaf, the super water-repellency of a water strider's leg, and the reduced water resistance of a shark's skin. Man-made applications include the application of a plastic coating with hierarchical surface topology on aircraft for drag reduction.
Currently available fabrication techniques include micro stereo lithography, a combination process of deep reactive ion etching and bulk micromachining, inclined deep X-ray lithography, and inclined ultraviolet lithography. All have demonstrated three dimensional microstructures and nanostructures. However, fabrication of hierarchical structures has not been possible. Most hierarchical structures reported were obtained from a self-assembly method. However, the fidelity of such structures and their long-range order are poor. These techniques are also low in throughput.
Since the publication by Chou et. al Stephen Y. Chou, Peter R. Krauss, Preston J. Renstrom, Appl. Phys. Lett 67 (1995) 3114, nanoimprint lithography (“NIL”) has been recognized as an attractive technique particularly for fabrication of two dimensional nanostructures. The primary working principle of nanoimprint lithography relies on the viscoelastic properties of polymers. In this way, a polymer film is heated to above its glass transition temperature (Tg). The polymer will then flow and acquire the topology of a hard mould. The pattern is set when the polymer is cooled to its glassy state. Therefore, the pattern resolution is primarily determined by the hard mould.